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[Xen-devel] Re: Questions about VIRT_BASE and ELF_PADDR_OFFSET in __xen_

To: "Puthiyaparambil, Aravindh" <aravindh.puthiyaparambil@xxxxxxxxxx>
Subject: [Xen-devel] Re: Questions about VIRT_BASE and ELF_PADDR_OFFSET in __xen_guest
From: Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>
Date: Thu, 18 May 2006 21:35:16 +0100
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On 18 May 2006, at 19:15, Puthiyaparambil, Aravindh wrote:

I was looking at your changeset:

I am finding that by making VIRT_BASE=0x0 is causing the builder to seg
fault. The same thing happens when VIRT_BASE and ELF_PADDR_OFFSET is set
to 0. I don't understand why dsi->elf_paddr_offset is being set to
VIRT_BASE and then overridden with ELF_PADDR_OFFSET if it is set. Does
ELF_PADDR_OFFSET have any real use? Does it correlate with anything in
the ELF program header? I am a little confused here :-), so could you
please explain.

VIRT_BASE is the linear address you want your memory allocation mapped at.

ELF_PADDR_OFFSET is a value subtracted from addresses in the Elf headers to produce a pseudo-physical address. We usually set it to VIRT_BASE since we write virtual (linear) addresses into those Elf header fields. The default of setting it to VIRT_BASE is simply to maintain backward compatibility with old guest images which do not explicitly define it.

There is no reason really why VIRT_BASE=0 should not work. If it crashes there is presumably some underlying bug which disallowing VIRT_BASE=0 does not fix.

 -- Keir

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